Many studies comparing different sources of lighting to grow strawberries have found that smart LEDs guarantee the best results. They produce a marked improvement in the performance of the plant in all the different stages of its life-cycle.
The strawberry we see in our markets or grow in our gardens is a hybrid of two species and is scientifically called Fragaria x ananassa . These fruits are not only tasty, but they are very healthy since they are low in fat and have a high content of beneficial vitamin-C, folic acid, potassium, and fiber (1).
Year Round Strawberry Production
A scientific study has found that low light and temperature, humidity fluctuations, and nutrition are the reasons hampering forced growing of strawberries (2). Beside experiments and trials, smart LEDs have also been tested in practical large-scale production. With suitable light recipes in addition to proper temperature and plant nutrition, it is possible to grow high quality strawberries even in winter (3).
In the absence of specific recommendations for germinating strawberry seeds, the general practice of using blue and red LEDs can be followed to break seed dormancy and facilitate germination of the seeds. Since strawberries like full sunlight, avoid the use of far-red which to plants signals shade (4).
In the case of strawberries, intensity, and type of light source is more important than the colour of light.
A study compared the use of fluorescent lights and LEDs of the same power to force strawberries, in comparison to natural lights (5). Under LEDs, photosynthesis is nearly six times higher than the plants grown under fluorescent light, and 10 times better than plants grown under natural lights. Maximum photosynthesis is observed when the supplemental light intensity is supplied at a PPFD of 400 μmol m-2 s-1. Increases in photosynthesis occur above this intensity levels but are minimal between 800 -1200 μmol m-2 s-1 (5).
This study found that light penetration and distribution is better under LEDs. The light is also four times higher under LEDs at all heights of the strawberry plants from ground level to its maximum height. So, at 10, 20, and 30 cm of plant heights, LEDs provide PPFD of 400, 690, and 1200 μmol m-2 s-1, respectively. Thus,they provide the necessary amounts of light intensity necessary for maximising photosynthesis (5).
Differences in the photosynthetic rate seen between plants grown under the three kinds of lighting are reflected in the overall plant growth, which is best under LEDs. Plants illuminated by LEDs have leaves, crowns, and roots that are 2.5, 2, and 1.6 times higher than plants grown under natural light. The leaf area of LED plants is also 30% more than those under fluorescent light (5).
As blue and red light control the rates of photosynthesis, LEDs of this color are the best to promote the vegetative growth of strawberry plants (6).
The reason for the enhanced yield under LEDs is the significant improvement in the photosynthetic rate it produces. The resulting increase in levels of carbohydrates stimulates flower bud formation and the subsequent development of inflorescences. So this provides a major starting point to improve yield (5).
Increasing Fruit Yield
LEDs can be used to manipulate strawberry fruit yield in different ways. Light color, intensity, and position are all important.
The use of single-spectral blue or red LEDs can significantly improve the yield of strawberry fruits when compared to fluorescent or natural light (6). Using a red and blue combination can also improve yield (7).
Using full-spectrum LEDs instead of fluorescent light to supplement natural light produces 1.3 - 1.5 times more of fruits. Similarly, the weight of the fruits grown on the LEDs is also 1.3 - 1.5 times more than those grown with fluorescent lights. So ultimately the marketable yield of strawberries grown under LEDs can be 1.8 - 2.4 times higher than those grown with fluorescent lights (5).
The increase in photosynthetic rate under LEDs allows higher production of carbohydrates. Since the strawberry plants, fortunately, make the greatest allocation of carbohydrates towards the formation of fruits, their boosted levels under LEDs, gives significantly more and larger fruits than other light sources (5).
Harvesting Time is Reduced
By using LEDs as top-lighting, many greenhouse growers have shortened time to harvest by 10 to 12 weeks, making strawberries available before the general peak season (5, 8). This, of course, translates to more profits (8). Moreover, LED inter-lighting with intensities of 57 W m-2, can also accelerate fruit maturation (6).
Tastier and Nutritious Strawberries
Position, intensity, and color of light can be used to manipulate the quality and nutritional value of strawberries.
LEDs in different positions and of varying intensities were tested in a study in Wageningen University and Research, to illuminate plants. In some cases, LEDs were used only to supplement natural light in the greenhouse. In other cases, the conditions duplicated those of indoor farming, and there was more use of LEDs - they were hung just 30 cm about the plants. In the third case, intermediary lightswere used in addition to top-lighting was so that light was also directed towards the developing fruits (9).
The whole strawberry plants benefit when they receive increased amounts of light. So the third model provided the best results, and the lights focused on fruits also improved their nutritional value and flavour. The juiciness, firmness, and vitamin-C content of fruits grown with LEDs were found to have improved (9).
20–40 mol m-2 s-1 of blue light increases the production of organic acids, and anthocyanin (the chemical which gives strawberry its characteristic red color). Green LEDs at 200 mol m-2 s-1 of green light also increase the production of vitamin-C, alpha-tocopherol, anthocyanin, and phenolic content (6).
In general, either blue or a combination or red and blue can be used to improve the nutritional value of crops (6).
Indoor vs. Greenhouse Conditions
One study reported the yield of fruits produced was higher in greenhouses where single spectral blue and red, or a combination of the two was used as supplements, compared to growth chambers where they were used as the sole source of lights. Similarly, there was more production of phenolic compounds in fruits grown under natural light supplemented with LEDs than with just colored LEDs (7).
So, indoor farmers should consider using white LEDS with red/ blue or a red and blue combination as supplements to duplicate greenhouse conditions.
Resistance to Diseases
Green light used with an intensity of 80 mol m-2 s-1, develops resistance in strawberry plants against Glomerella cingulate (6)
It is quite common for fruits to plucked before they are completely ripe so that they can be transported for a long-distance or stored for a longer time before they are eventually sold. The quality of strawberries can be preserved and regulated even after they are harvested.
Color of Fruits
Using blue LEDs can enhance anthocyanins levels in immature strawberries in just 3 to 4 days. Supplemental blue light used at an intensity of 40 μmol m(-2) s(-1) at 5°C increases the levels of anthocyanin content post- harvest (10). However, in strawberries, blue LEDs also quickens the ripening of fruit by increasing respiration and production of ethylene (11).
If rapid ripening of fruits is not desirable postharvest, continuous exposure to green LEDs can increase the level of anthocyanins and intensify the red colour of strawberries to improve its aesthetics (11).
The development of aroma and taste in immature strawberry fruits can be continued even after they are harvested by using narrow bandwidth LEDs. This happens due to the accumulation of compounds such as benzenoid and phenylpropanoid. Thus, it is better to store strawberry fruit using LEDs instead of fluorescent white light or keeping them in the dark (6).
Fruit can also be kept fresher when they are stored under red LEDs as this prevents the loss of moisture. This helps them to remain more firm, and look better than when stored in the dark (6).
LEDs can prevent infection of post-harvest fruits during storage. Blue LEDs prevent mold growth, brought about by Botrytis cinerea. Strawberries stored in the dark, on the other hand, can get covered by this mold under the same environmental conditions (11). By improving fruit storage through LEDs, it is possible to reduce the use of chemicals and make the strawberries more safe for human consumption (11).
LEDs as Part of the Strategy
The benefit from smart horticulture LED lights that strawberry plants derive is mostly through an increase in intensity. In addition, the position of lights increases yield, flavour and shortens harvest time.
- Miyoshi Y, Hidaka K, Okayasu T, et al. 2013. Approach to Local Environment Control for Stable Production of Strawberry. IFAC Proceedings Volumes 46: 58-61.https://doi.org/10.3182/20130327-3-JP-3017.00016
Hidaka K, Dan K, Imamura H, et al. 2013. Investigation of Supplemental Lighting with Different Light Source for High Yield of Strawberry. IFAC Proceedings Volumes
46: 115-119. https://doi.org/10.3182/20130327-3-JP-3017.00028
- Hasan MM et al. 2017. An Overview of LEDs’ Effects on the Production ofBioactive Compounds and Crop Quality. Molecules 22: 1420. doi:10.3390/molecules22091420
- Choi HG, Moon BY, and NJ Kang. Effects of LED light on the production of strawberry during cultivation in a plastic greenhouse and in a growth chamber. Sci. Hortic. 2015, 189, 22–31. https://doi.org/10.1016/j.scienta.2015.03.022
- Xu F, Cao SF, Shi LY, et.-al. 2014.. Blue light irradiation affects anthocyanin content and enzyme activities involved in postharvest strawberry fruit. J. Agric. Food Chem. 62: 4778–4783.
- D’Souza C, Yuk HG, Khoo GH, and W Zhou. 2015. Application of light-emitting diodes in food production, postharvest preservation, and microbiological food safety. Compr. Rev. Food Sci. Food Saf. 14:719–740.